% ==========================================================================
% $Id: letter.pl,v 1.1 2014/02/04 03:57:03 jlang Exp $
% CSI2120 example Code for lecture 8
% ==========================================================================
% (C)opyright:
%
%   Jochen Lang
%   EECS, University of Ottawa
%   800 King Edward Ave.
%   Ottawa, On., K1N 6N5
%   Canada. 
%   http://www.eecs.uottawa.ca/~jlang
% 
% Creator: jlang (Jochen Lang) based on notes by R. Laganiere
% Email:   jlang@eecs.uottawa.ca
% ==========================================================================
% $Log: letter.pl,v $
% Revision 1.1  2014/02/04 03:57:03  jlang
% Added initial example for database manipulations.
%
% ==========================================================================
:- dynamic letter/2.

alphabet([a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z]).

% add a rule for each letter queried
letter(A, B):-
  alphabet(C),
  letter(A, C, B),
  asserta(letter(A,B)). % Add a new fact

letter(A, [A|_], 1). % boundary case, letter matches

letter(A, [B|C], D):- 
  \+(A=B), % letter does not match, keep searching
  letter(A, C, E),
  D is E+1. % Count on the way out of the recursion


% ?- letter(h,X).